Blood clots consist of a fibrous network that is capable of dissolution by the proteolytic enzyme, plasmin. The enzyme is derived from the inactive proenzyme, plasminogen, a component of blood plasma, by the action of a plasminogen activator. There are two immunologically distinct mammalian plasminogen activators. Intrinsic plasminogen activator, also known as urokinase, is an enzyme produced by the kidney and can be isolated from urine. It can also be prepared from a number of tissue culture sources. Extrinsic plasminogen activator, also known as vascular plasminogen activator and as tissue plasminogen activator (t-PA), can be isolated from many tissue homogenates (notably human uterus), the vascular cell wall and from some cell cultures. In addition to these two kinds of plasminogen activator, there is also a bacterial product, streptokinase (streptokinase), prepared from streptococci. 
With the escalating use of arterial and venous catheters in the clinics, locally delivered active plasmin offers an attractive therapeutic opportunity in thrombolytic therapy or opening clogged catheters. There are a number of reasons for this: 1) Being an active serine protease, plasmin is a direct clot dissolving agent in contrast to plasminogen activators, which require the presence of the substrate (plasminogen) in the vicinity of the clot; 2) Local catheter directed thrombolytic therapy with active plasmin can be intensified to whatever level is required to achieve completeness of clot lysis; 3) Plasmin also has the theoretical potential to be a safer thrombolytic because the lower dosage required for local delivery may decrease or even eliminate bleeding complications associated with high dose thrombolytic therapy and any potential spillage of plasmin activity from the immediate vicinity of the thrombus site will be quickly neutralized by circulating α2-antiplasmin.
There are several technical challenges associated with plasmin purification, especially with its therapeutic use and delivery. Plasmin is an active serine protease which is prone to autodigestion and inactivation at physiological pH. Unfortunately, plasmin degradation is most noticeable in the pH range required for manifestation of its function, clot lysis.
Current processes for commercial activation of plasma-derived plasminogen to plasmin employ soluble streptokinase in a reaction carried out in the liquid phase. The plasmin product of this activation reaction is not fully stabilized against self-proteolysis until the activation step has proceeded to the desired extent of conversion of plasminogen to plasmin. During this activation, streptokinase is cleaved by plasmin, necessitating the removal of multiple molecular species of streptokinase from the final product. Further, newly formed plasmin molecules can also begin cleaving other plasmin/plasminogen molecules, resulting in loss of valuable product, i.e., plasmin.
Thus, there is presently a need for simple and efficient methods or processes to prepare plasmin. It is additionally desirable that such a method provides plasmin solutions substantially free of the streptokinase, such that, if desired, the plasmin can be used for administering (e.g., parenterally) as a pharmaceutical.